Control system for regenerative braking in a hybrid vehicle

ABSTRACT

A vehicle includes a powertrain, an electric machine, a battery, and a controller. The powertrain is configured to transfer motive power to the electric machine to charge the battery during regenerative braking. The controller programmed to, in response to a decreasing demanded powertrain output torque, adjust a regenerative braking torque limit based on an anti-jerk torque schedule, generate an actual regenerative braking torque based on system constraints, and limit the actual regenerative braking torque to the regenerative braking torque limit.

TECHNICAL FIELD

The present disclosure relates to hybrid vehicles and methods of controlling regenerative braking.

BACKGROUND

Regenerative braking is a feature of hybrid vehicles that improves fuel economy by recapturing kinetic energy when the vehicle slows down during a braking event. During regenerative braking, an electric motor may operate as a generator to convert the motive energy of the vehicle into electrical energy which is in turn used to charge a battery.

SUMMARY

A vehicle includes a controller. The controller is programmed to, in response to wheel torque becoming negative in the presence of an increasing braking request while a battery state of charge is such that regenerative braking alone could satisfy the braking request, limit regenerative braking according to a wheel torque schedule such that the regenerative braking does not satisfy the braking request, and increase friction braking to satisfy a remainder of the braking request.

A vehicle system includes a controller. The controller is programmed to, while demanded wheel torque is decreasing, limit actual regenerative braking torque based on an available regenerative braking torque that adjusts according to a torque schedule such that the actual regenerative braking does not satisfy an increasing braking request while a battery state of charge is such that regenerative braking alone could satisfy the braking request, and increase friction braking to satisfy a remainder of the braking request

A vehicle includes a powertrain, an electric machine, a battery, and a controller. The powertrain is configured to transfer motive power to the electric machine to charge the battery during regenerative braking. The controller programmed to, in response to a decreasing demanded powertrain output torque, adjust a regenerative braking torque limit based on an anti-jerk torque schedule, generate an actual regenerative braking torque based on system constraints, and limit the actual regenerative braking torque to the regenerative braking torque limit.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic illustration of an exemplary powertrain of a hybrid electric vehicle;

FIG. 2 is flowchart illustrating a vehicle powertrain controller;

FIG. 3 is a flowchart illustrating a wheel torque generating unit of the powertrain controller;

FIG. 4 is a flowchart illustrating an interface between a vehicle control unit of the powertrain controller and a brake control unit of the powertrain controller; and

FIG. 5 is exemplary graph illustrating the relationships between wheel torque, regenerative braking torque, and friction braking torque.

DETAILED DESCRIPTION

Embodiments of the present disclosure are described herein. It is to be understood, however, that the disclosed embodiments are merely examples and other embodiments may take various and alternative forms. The figures are not necessarily to scale; some features could be exaggerated or minimized to show details of particular components. Therefore, specific structural and functional details disclosed herein are not to be interpreted as limiting, but merely as a representative basis for teaching one skilled in the art to variously employ the present invention. As those of ordinary skill in the art will understand, various features illustrated and described with reference to any one of the figures may be combined with features illustrated in one or more other figures to produce embodiments that are not explicitly illustrated or described. The combinations of features illustrated provide representative embodiments for typical applications. Various combinations and modifications of the features consistent with the teachings of this disclosure, however, could be desired for particular applications or implementations.

Referring to FIG. 1, a schematic diagram of a hybrid electric vehicle (HEV) 10 is illustrated according to an embodiment of the present disclosure. FIG. 1 illustrates representative relationships among the components. Physical placement and orientation of the components within the vehicle may vary. The HEV 10 includes a powertrain 12. The powertrain 12 includes an engine 14 that drives a transmission 16, which may be referred to as a modular hybrid transmission (MHT). As will be described in further detail below, transmission 16 includes an electric machine such as an electric motor/generator (M/G) 18, an associated traction battery 20, a torque converter 22, and a multiple step-ratio automatic transmission, or gearbox 24.

The engine 14 and the M/G 18 are both drive sources for the HEV 10. The engine 14 generally represents a power source that may include an internal combustion engine such as a gasoline, diesel, or natural gas powered engine, or a fuel cell. The engine 14 generates an engine power and corresponding engine torque that is supplied to the M/G 18 when a disconnect clutch 26 between the engine 14 and the M/G 18 is at least partially engaged. The M/G 18 may be implemented by any one of a plurality of types of electric machines. For example, M/G 18 may be a permanent magnet synchronous motor. Power electronics condition direct current (DC) power provided by the battery 20 to the requirements of the M/G 18, as will be described below. For example, power electronics may provide three phase alternating current (AC) to the M/G 18.

When the disconnect clutch 26 is at least partially engaged, power flow from the engine 14 to the M/G 18 or from the M/G 18 to the engine 14 is possible. For example, the disconnect clutch 26 may be engaged and M/G 18 may operate as a generator to convert rotational energy provided by a crankshaft 28 and M/G shaft 30 into electrical energy to be stored in the battery 20. The disconnect clutch 26 can also be disengaged to isolate the engine 14 from the remainder of the powertrain 12 such that the M/G 18 can act as the sole drive source for the HEV 10. Shaft 30 extends through the M/G 18. The M/G 18 is continuously drivably connected to the shaft 30, whereas the engine 14 is drivably connected to the shaft 30 only when the disconnect clutch 26 is at least partially engaged.

The M/G 18 is connected to the torque converter 22 via shaft 30. The torque converter 22 is therefore connected to the engine 14 when the disconnect clutch 26 is at least partially engaged. The torque converter 22 includes an impeller fixed to M/G shaft 30 and a turbine fixed to a transmission input shaft 32. The torque converter 22 thus provides a hydraulic coupling between shaft 30 and transmission input shaft 32. The torque converter 22 transmits power from the impeller to the turbine when the impeller rotates faster than the turbine. The magnitude of the turbine torque and impeller torque generally depend upon the relative speeds. When the ratio of impeller speed to turbine speed is sufficiently high, the turbine torque is a multiple of the impeller torque. A torque converter bypass clutch (also known as a torque converter lock-up clutch) 34 may also be provided that, when engaged, frictionally or mechanically couples the impeller and the turbine of the torque converter 22, permitting more efficient power transfer. The torque converter bypass clutch 34 may be operated as a launch clutch to provide smooth vehicle launch. Alternatively, or in combination, a launch clutch similar to disconnect clutch 26 may be provided between the M/G 18 and gearbox 24 for applications that do not include a torque converter 22 or a torque converter bypass clutch 34. In some applications, disconnect clutch 26 is generally referred to as an upstream clutch and launch clutch 34 (which may be a torque converter bypass clutch) is generally referred to as a downstream clutch.

The gearbox 24 may include gear sets (not shown) that are selectively placed in different gear ratios by selective engagement of friction elements such as clutches and brakes (not shown) to establish the desired multiple discrete or step drive ratios. The friction elements are controllable through a shift schedule that connects and disconnects certain elements of the gear sets to control the ratio between a transmission output shaft 36 and the transmission input shaft 32. The gearbox 24 is automatically shifted from one ratio to another based on various vehicle and ambient operating conditions by an associated controller, such as a powertrain control unit (PCU). Power and torque from both the engine 14 and the M/G 18 may be delivered to and received by gearbox 24. The gearbox 24 then provides powertrain output power and torque to output shaft 36.

It should be understood that the hydraulically controlled gearbox 24 used with a torque converter 22 is but one example of a gearbox or transmission arrangement; any multiple ratio gearbox that accepts input torque(s) from an engine and/or a motor and then provides torque to an output shaft at the different ratios is acceptable for use with embodiments of the present disclosure. For example, gearbox 24 may be implemented by an automated mechanical (or manual) transmission (AMT) that includes one or more servo motors to translate/rotate shift forks along a shift rail to select a desired gear ratio. As generally understood by those of ordinary skill in the art, an AMT may be used in applications with higher torque requirements, for example.

As shown in the representative embodiment of FIG. 1, the output shaft 36 is connected to a differential 40. The differential 40 drives a pair of wheels 42 via respective axles 44 connected to the differential 40. The differential transmits approximately equal torque to each wheel 42 while permitting slight speed differences such as when the vehicle turns a corner. Different types of differentials or similar devices may be used to distribute torque from the powertrain to one or more wheels. In some applications, torque distribution may vary depending on the particular operating mode or condition, for example.

The powertrain 12 further includes an associated controller 50 such as a powertrain control unit (PCU). While illustrated as one controller, the controller 50 may be part of a larger control system and may be controlled by various other controllers throughout the vehicle 10, such as a vehicle system controller (VSC). It should therefore be understood that the powertrain control unit 50 and one or more other controllers can collectively be referred to as a “controller” that controls various actuators in response to signals from various sensors to control functions such as starting/stopping engine 14, operating M/G 18 to provide wheel torque or charge battery 20, select or schedule transmission shifts, etc. Controller 50 may include a microprocessor or central processing unit (CPU) in communication with various types of computer readable storage devices or media. Computer readable storage devices or media may include volatile and nonvolatile storage in read-only memory (ROM), random-access memory (RAM), and keep-alive memory (KAM), for example. KAM is a persistent or non-volatile memory that may be used to store various operating variables while the CPU is powered down. Computer-readable storage devices or media may be implemented using any of a number of known memory devices such as PROMs (programmable read-only memory), EPROMs (electrically PROM), EEPROMs (electrically erasable PROM), flash memory, or any other electric, magnetic, optical, or combination memory devices capable of storing data, some of which represent executable instructions, used by the controller in controlling the engine or vehicle.

The controller communicates with various engine/vehicle sensors and actuators via an input/output (I/O) interface (including input and output channels) that may be implemented as a single integrated interface that provides various raw data or signal conditioning, processing, and/or conversion, short-circuit protection, and the like. Alternatively, one or more dedicated hardware or firmware chips may be used to condition and process particular signals before being supplied to the CPU. As generally illustrated in the representative embodiment of FIG. 1, controller 50 may communicate signals to and/or from engine 14, disconnect clutch 26, M/G 18, battery 20, launch clutch 34, transmission gearbox 24, and power electronics 56. Although not explicitly illustrated, those of ordinary skill in the art will recognize various functions or components that may be controlled by controller 50 within each of the subsystems identified above. Representative examples of parameters, systems, and/or components that may be directly or indirectly actuated using control logic and/or algorithms executed by the controller include fuel injection timing, rate, and duration, throttle valve position, spark plug ignition timing (for spark-ignition engines), intake/exhaust valve timing and duration, front-end accessory drive (FEAD) components such as an alternator, air conditioning compressor, battery charging or discharging (including determining the maximum charge and discharge power limits), regenerative braking, M/G operation, clutch pressures for disconnect clutch 26, launch clutch 34, and transmission gearbox 24, and the like. Sensors communicating input through the I/O interface may be used to indicate turbocharger boost pressure, crankshaft position (PIP), engine rotational speed (RPM), wheel speeds (WS1, WS2), vehicle speed (VSS), coolant temperature (ECT), intake manifold pressure (MAP), accelerator pedal position (PPS), ignition switch position (IGN), throttle valve position (TP), air temperature (TMP), exhaust gas oxygen (EGO) or other exhaust gas component concentration or presence, intake air flow (MAF), transmission gear, ratio, or mode, transmission oil temperature (TOT), transmission turbine speed (TS), torque converter bypass clutch 34 status (TCC), deceleration or shift mode (MDE), battery temperature, voltage, current, or state of charge (SOC) for example.

Control logic or functions performed by controller 50 may be represented by flow charts or similar diagrams in one or more figures. These figures provide representative control strategies and/or logic that may be implemented using one or more processing strategies such as event-driven, interrupt-driven, multi-tasking, multi-threading, and the like. As such, various steps or functions illustrated may be performed in the sequence illustrated, in parallel, or in some cases omitted. Although not always explicitly illustrated, one of ordinary skill in the art will recognize that one or more of the illustrated steps or functions may be repeatedly performed depending upon the particular processing strategy being used. Similarly, the order of processing is not necessarily required to achieve the features and advantages described herein, but is provided for ease of illustration and description. The control logic may be implemented primarily in software executed by a microprocessor-based vehicle, engine, and/or powertrain controller, such as controller 50. Of course, the control logic may be implemented in software, hardware, or a combination of software and hardware in one or more controllers depending upon the particular application. When implemented in software, the control logic may be provided in one or more computer-readable storage devices or media having stored data representing code or instructions executed by a computer to control the vehicle or its subsystems. The computer-readable storage devices or media may include one or more of a number of known physical devices which utilize electric, magnetic, and/or optical storage to keep executable instructions and associated calibration information, operating variables, and the like.

An accelerator pedal 52 is used by the driver of the vehicle to provide a demanded torque, power, or drive command to propel the vehicle. In general, depressing and releasing the accelerator pedal 52 generates an accelerator pedal position signal that may be interpreted by the controller 50 as a demand for increased power or decreased power, respectively. A brake pedal 58 is also used by the driver of the vehicle to provide a demanded braking torque to slow the vehicle. In general, depressing and releasing the brake pedal 58 generates a brake pedal position signal that may be interpreted by the controller 50 as a demand to decrease the vehicle speed. Based upon inputs from the accelerator pedal 52 and brake pedal 58, the controller 50 commands the torque to the engine 14, M/G 18, and friction brakes 60. The controller 50 also controls the timing of gear shifts within the gearbox 24, as well as engagement or disengagement of the disconnect clutch 26 and the torque converter bypass clutch 34 Like the disconnect clutch 26, the torque converter bypass clutch 34 can be modulated across a range between the engaged and disengaged positions. This produces a variable slip in the torque converter 22 in addition to the variable slip produced by the hydrodynamic coupling between the impeller and the turbine. Alternatively, the torque converter bypass clutch 34 may be operated as locked or open without using a modulated operating mode depending on the particular application.

To drive the vehicle with the engine 14, the disconnect clutch 26 is at least partially engaged to transfer at least a portion of the engine torque through the disconnect clutch 26 to the M/G 18, and then from the M/G 18 through the torque converter 22 and gearbox 24. The M/G 18 may assist the engine 14 by providing additional power to turn the shaft 30. This operation mode may be referred to as a “hybrid mode” or an “electric assist mode.”

To drive the vehicle with the M/G 18 as the sole power source, the power flow remains the same except the disconnect clutch 26 isolates the engine 14 from the remainder of the powertrain 12. Combustion in the engine 14 may be disabled or otherwise OFF during this time to conserve fuel. The traction battery 20 transmits stored electrical energy through wiring 54 to power electronics 56 that may include an inverter, for example. The power electronics 56 convert DC voltage from the battery 20 into AC voltage to be used by the M/G 18. The controller 50 commands the power electronics 56 to convert voltage from the battery 20 to an AC voltage provided to the M/G 18 to provide positive or negative torque to the shaft 30. This operation mode may be referred to as an “electric only” or “EV” operation mode.

In any mode of operation, the M/G 18 may act as a motor and provide a driving force for the powertrain 12. Alternatively, the M/G 18 may act as a generator and convert kinetic energy from the powertrain 12 into electric energy to be stored in the battery 20. The M/G 18 may act as a generator while the engine 14 is providing propulsion power for the vehicle 10, for example. The M/G 18 may additionally act as a generator during times of regenerative braking in which torque and rotational (or motive) energy or power from spinning wheels 42 is transferred back through the gearbox 24, torque converter 22, (and/or torque converter bypass clutch 34) and is converted into electrical energy for storage in the battery 20.

It should be understood that the schematic illustrated in FIG. 1 is merely exemplary and is not intended to be limiting. Other configurations are contemplated that utilize selective engagement of both an engine and a motor to transmit through the transmission. For example, the M/G 18 may be offset from the crankshaft 28, an additional motor may be provided to start the engine 14, and/or the M/G 18 may be provided between the torque converter 22 and the gearbox 24. Other configurations are contemplated without deviating from the scope of the present disclosure.

During regenerative braking, it may be advantageous for the control system of the HEV 10 to coordinate the operation of the powertrain and braking systems to maximize fuel economy while also accounting for vehicle drivability. This may be accomplished by adapting the control systems to consider a wheel torque schedule, which may include anti-jerk control, during a regenerative braking event. Failing to consider the wheel torque schedule during regenerative braking may lead to torque holes during braking because the brake control is not aware of the actual status of the powertrain. This may also results in the transmission unnecessarily capacitizing the torque converter to handle more negative torques when the powertrain has not actually requested so, causing waste of energy.

Referring to FIG. 2, the controller (or powertrain control unit) 50 is illustrated. Various functions of the controller 50 may be implemented via algorithms and/or control logic that are stored within the memory of the controller 50. The controller 50 includes a vehicle control unit 62 that is configured to generate a torque request to the wheels 42. The vehicle control unit 62 communicates with a brake control unit 64 and a transmission control unit 66. The transmission control unit 66 may refer to portion of the controller 50 that is used to control the transmission gearbox 24, the torque converter 22, and/or the torque converter bypass clutch 34. The brake control unit 64 may refer to the portion of the controller 50 that is used to control either the friction brakes 60 or the M/G 18 during regenerative braking. The controller 50 may include a series of input channels and output channels to communicate with and control various components of the HEV 10. Signals indicative of various states of the vehicle may be received by the controller 50 via the input channels and signals indicative of commands may be sent from the controller 50 to various components of the HEV 10. The controller 50 may also include internal input and output channels such that the various components (described in further detail below) that comprise the controller 50 may communicate with each other.

The vehicle control unit 62 includes a wheel torque generating unit (or powertrain output torque generating unit) 68 that generates a constrained wheel torque request (or constrained powertrain output torque request). The wheel torque generating unit 68 transmits a signal indicative of the constrained wheel torque request to an interface 70 from the vehicle control unit 62 to the transmission control unit 66. The interface 70 generates a powertrain braking torque request. The interface 70 then transmits a signal indicative of the powertrain braking torque request to the transmission control unit 66. The transmission control unit 66 in turn transmits a feedback signal indicative of a transmission regenerative braking torque limit to the wheel torque generating unit 68. The transmission regenerative braking torque limit may refer to the torque capacity of the transmission gearbox 24, the torque converter 22, and the torque converter bypass clutch 34 depending on the current operating mode (e.g., is the bypass clutch 34 currently open, slipping, or locked), the current gear selection in the gearbox 24, and/or the stress/strain limits of the physical components within the transmission gearbox 24, the torque converter 22, and the torque converter bypass clutch 34 during a regenerative braking event.

The wheel torque generating unit 68 also transmits a signal indicative of the constrained wheel torque request to an interface 72 from the vehicle control unit 62 to the brake control unit 64. The interface 72 generates an available powertrain regenerative braking torque (or powertrain regenerative braking torque limit). The interface 72 then transmits a signal indicative of the available powertrain regenerative braking torque to the brake control unit 64. The brake control unit 64 in turn transmits a signal indicative of a vehicle stability torque limit to the wheel torque generating unit 68. The vehicle stability torque limit may refer to a maximum regenerative braking torque that can be applied on the wheels without causing a vehicle yaw instability event.

Referring to FIG. 3, the wheel torque generating unit 68 of the controller 50 is illustrated. The wheel torque generating unit 68 includes a wheel torque request determination unit (or powertrain output torque request determination unit) 74 and a wheel torque shaping unit (or powertrain output torque shaping unit) 76. The wheel torque request determination unit 74 includes control logic and/or algorithms that are used to determine an actual wheel output torque request (or actual powertrain output torque request) based on inputs including the transmission regenerative braking torque limit, the vehicle stability torque, a charging limit of the battery 20, a torque limit of the M/G 18, a torque request based on the position of the accelerator pedal 52, and a braking torque request based on the position of the brake pedal 58. Once the actual wheel torque request is determined, the wheel torque request determination unit 74 transmits a signal indicative of the actual wheel torque request to the wheel torque shaping unit 76. The wheel torque shaping unit 76 includes control logic and/or algorithms that are used to generate the constrained wheel torque request. The constrained wheel torque request is a modified, altered, or filtered torque request based the actual wheel or actual powertrain output torque request from the vehicle user. For example, the constrained wheel torque request may produce a wheel torque that deviates from an actual wheel torque request of the vehicle user to either increase efficiency or improve drivability of the HEV 10. For example, the constrained wheel torque request may be based on a wheel or powertrain torque schedule, such as an anti-jerk torque schedule, to prevent any NVH issues from occurring within the vehicle powertrain or other components of the drivetrain at the time of changing gears, performing a tip-in/out, braking, etc. The other components of the drivetrain may include the torque converter 22, gearbox 24, output shaft 36, differential 40, and axles 44.

Referring FIG. 4, the interface 72 between the vehicle control unit 62 and the brake control unit 64 of the controller 50 is illustrated. FIG. 4 also illustrates the how the brake control unit 64 calculates a friction brake torque request during a regenerative braking event. The interface 72 and brake control unit 64 include control logic and/or algorithms that perform the various functions of the interface 72 and brake control unit 62 that are described below.

The interface 72 includes a first input block 78, a second input block 80, a third input block 82, a maximum block 84, and an arbitration block 86. The first input block 78 transmits a signal indicative of the transmission regenerative braking torque limit (which is generated by the transmission control unit 66) to the maximum block. The second input block 80 transmits a signal indicative of the torque limit of the M/G 18 (which may be limited by the capacity of the battery 20 receive charge) to the maximum block 84. The maximum block 84 determines the maximum of the transmission regenerative braking torque limit and the torque limit of the M/G 18 (taking into account the charge limit of the battery 20). Next, the maximum block 84 transmits a signal indicative of the maximum of transmission regenerative braking torque limit and torque limit of the M/G 18 (taking into account the charge limit of the battery 20) to the arbitration block 86. The third input block 82 transmits a signal indicative of the constrained wheel torque request (which is generated by the wheel torque generating unit 68) to the arbitration block 86. The arbitration block 86 then transmits a signal indicative of the available powertrain regenerative braking torque to the brake control unit 64.

The brake control unit 64 includes a first input block 88, a second input block 90, a third input block 92, a fourth input block 94, and a friction brake torque request calculation block 96. The first input block 88 transmits a signal indicative of the available powertrain regenerative braking torque to the friction brake torque request calculation block 96. The available powertrain regenerative braking is received by the first input block from the arbitration block 86 of the interface 72 between the vehicle control unit 62 and the brake control unit 64. The second input block 90 transmits a signal indicative of the actual powertrain wheel torque (or actual powertrain output torque) to the friction brake torque request calculation block 96. The actual powertrain real torque may be received from the vehicle control unit 62 or maybe transmitted from a sensor that is configured to detect the torque at the wheels 42 (or the output of the powertrain). The third input block 92 transmits a signal indicative of a total brake torque demand to the to the friction brake torque request calculation block 96. The total brake torque demand may be based on a position of the brake pedal 58 caused by a user is depressing the brake pedal 58. The fourth input block 94 transmits a signal indicative of the vehicle stability torque limit to the to the friction brake torque request calculation block 96. The vehicle stability torque limit may refer to a maximum regenerative braking torque that can be applied on the wheels without causing a vehicle yaw instability event. The information input into the friction brake torque request calculation block 96 is transformed into a friction brake torque request based on control logic and/or algorithms that are included within the friction brake torque request calculation block 96. The friction brake torque request calculation block 96 then transmits a signal indicative of the friction brake torque request to the wheels 42.

Referring to FIG. 5, an exemplary graph representing the relationships between wheel torque, regenerative braking torque, and friction braking torque is illustrated. The relationships represented in the graph are based on the functionality of the controller 50 including the functionality of, but not limited, the interface 72 and the brake control unit 64. A first plot on the graph represents an unfiltered demanded wheel torque (or unfiltered demanded powertrain output torque) 98 plotted against time. The unfiltered demanded wheel torque 98 may be based on the torque commands received from the user of the HEV 10 via the accelerator pedal 52 and/or brake pedal 58. A second plot on the graph represents a filtered demanded wheel torque (or filtered demanded powertrain output torque) 100 plotted against time. The filtered demanded wheel torque may correspond to the constrained wheel torque request. The unfiltered demanded wheel torque 98 may be filtered according to a wheel torque schedule to coordinate the operation of the powertrain and braking systems to maximize fuel economy or improve vehicle drivability. For example, the filtered demanded wheel torque 100 may follow a torque schedule that slowly decreases the wheel (or powertrain output) torque when there is a sudden change from a demanded wheel torque having a positive value to a negative value in order to prevent NVH issues from occurring within the powertrain or other components of the drivetrain. The described example of the filtered demanded wheel torque 100 may be referred to as an anti-jerk torque control system or an anti-jerk torque schedule. A third plot on the graph represents the available powertrain regenerative braking torque (or powertrain regenerative braking torque limit) 102 plotted against time, which is also represented by the first input block 88 of the brake control unit 64 in FIG. 4. A fourth plot on the graph represents total brake torque demand 104 plotted against time, which is also represented by the third input block 92 brake control unit 64 in FIG. 4. A fifth plot on the graph represents the friction brake torque request 106 plotted against time, which is generated at the friction brake torque request calculation block 96 in FIG. 4.

When a vehicle user releases the accelerator pedal 52, which is represented as the event occurring at time t₁, the unfiltered demanded wheel torque 98 may quickly dropped to a demanded negative wheel torque value at time t₂. The negative wheel torque value at time t₂ represents the wheel torque value when the accelerator pedal 52 is completely released (which may be referred to as the lift pedal torque request) prior to receiving a braking torque request. The filtered demanded wheel torque request 100, however, will prevent the actual wheel torque from dropping as quickly as the unfiltered demanded wheel torque 98 to prevent NVH issues in order to improve the drivability of the HEV 10. The actual wheel torque may be approximately equal to the filtered demanded wheel torque request 100.

Shortly after the user releases the accelerator pedal 52, between times t₃ and t₄, the filtered demanded wheel torque 100 will experience a period of time where torque is decreased (or ramped down) to simulate a damping torque that occurs from engine compression. This period of time may be referred to as a damping (or dashpot) period 108. Once the damping period ends at time t₄, the filtered demanded wheel torque request 100 continues to decrease during a period of lash crossing 110. Lash crossing refers to a period of time where there is a change in direction in the torque being transferred through the powertrain and drivetrain. The rate at which the filtered demanded wheel torque 100 decreases during the period of lash crossing 110 is less than the rate at which the filtered demanded wheel torque 100 decreases during the damping period 108. The period of lash crossing 110 occurs between times t₄ and t₅. The period of lash crossing 110 may be divided into two periods, the first corresponding to positive filtered demanded wheel torque request values while the other corresponds to negative filtered demanded wheel torque request values. The portion of the period of lash crossing 110 where the filtered demanded wheel torque request 100 has positive values occurs between times t₄ and t₆. The portion of the period of lash crossing 110 where the filtered demanded wheel torque request 100 has negative values occurs between times t₆ and t₅. Once the period of lash crossing 110 is completed at time t₅, the rate at which the filtered demanded wheel torque request 100 decreases is increased until and the filtered demanded wheel torque request 100 is ramped to the torque value of the unfiltered demanded wheel torque 98 at time t₇. At time t₇, the filtered demanded wheel torque request 100 merges with the unfiltered demanded wheel torque 98 resulting in a single unfiltered demanded wheel torque.

When a vehicle user depresses the brake pedal 58 (which is demonstrated as occurring beginning at time t₈), the controller 50 will need to determine if it is possible to utilize the braking energy for some purpose or recapture the braking energy to charge the battery 20 with the M/G 18 via regenerative braking. First, the available powertrain regenerative braking torque 102 is determined based on the control logic and/or algorithms of the interface 72 from the vehicle control unit 62 to the brake control unit 64. As previously stated, the available powertrain regenerative braking torque 102 is based on the output of the arbitration block 86 in FIG. 4, which is an arbitration between the constrained wheel torque request and the maximum of the transmission regenerative braking torque limit and the torque limit of the M/G 18 (taking into account the charge limit of the battery 20). Again, the maximum of the transmission regenerative braking torque limit and the torque limit of the M/G 18 (taking into account the charge limit of the battery 20) is determined in the maximum block 84. For simplicity purposes, the output of the maximum block 84 will be referred to as the powertrain maximum regenerative braking torque. The process occurring within the arbitration block 86 will limit the available powertrain regenerative braking torque 102 to the torque schedule of the filtered demanded wheel torque 100, during a period decreasing demanded wheel torque until the filtered demanded wheel torque 100 merges with the unfiltered demanded wheel torque 98 at time t₇. During a first period from time t₈ to time t₆, when the filtered demanded wheel torque 100 is positive after the vehicle user depresses the brake pedal 58, the available powertrain regenerative braking torque 102 will be limited to zero. The first period, from time t₀ to time t₆, includes the damping period 108 and a first portion of the period of lash crossing 110, between times t₄ and t₆, corresponding to the portion of the period of lash crossing 110 where the filtered demanded wheel torque 100 is positive. Once the filtered demanded wheel torque 100 becomes negative during the period of lash crossing 110 at time t₆, the available powertrain regenerative braking torque 102 will be equal to the filtered demanded wheel torque 100 during a second period. The second period corresponds to a time period between t₆ (the filtered demanded wheel torque 100 becoming negative during the period of lash crossing 110) and t₅ (the completion of the period of lash crossing 110). Once the period of lash crossing is complete at time t₅, the available powertrain regenerative braking torque 102 is no longer limited by the wheel torque schedule of the filtered demanded wheel torque 100 and is be ramped to obtain the powertrain maximum regenerative braking torque at time t₇.

Also, when the vehicle user is depresses the brake pedal 58 at time t₈, the total brake torque demand 104 will be filled by the friction brake torque request 106 until the available powertrain regenerative braking torque 102 drops below the lift pedal torque request at time t₉. Once the available powertrain regenerative braking torque 102 drops below the negative value of the lift pedal torque request at time t₉, an actual regenerative braking torque (which is limited to available powertrain regenerative braking torque 102) will be requested and commanded (based on the total brake torque demand 104, and a need, demand, or request to recapture braking energy to charge the battery 20 or a need to utilize braking energy for some other purpose). The negative value of the friction brake torque request 106 will decrease by the amount of delivered actual regenerative braking torque. The actual regenerative braking torque will be limited by the constraints such as the transmission regenerative braking torque limit and the torque limit of the M/G 18 (which may be limited by the capacity of the battery 20 receive charge). In the shown example, the actual regenerative braking torque is represented by the line 107 and is the difference between the total brake torque demand 104 and the friction brake torque request 106. In the shown example, the actual regenerative braking torque 107 would not satisfy the brake torque request 106, during a period of an increasing brake torque request 106, if the friction brakes 60 were unavailable and battery state of charge was such that regenerative braking alone could satisfy the braking request, since the actual regenerative braking torque 107 is limited to the available powertrain regenerative braking torque 102. Between the time period t₉ and t₇, the negative value of the friction braking torque request 106 is decreased as the actual regenerative braking torque is increased. In the example depicted in FIG. 5, the negative friction braking torque request 106 and the actual regenerative braking torque are ramped to steady values at time t₇, however, it should be understood that the negative friction braking torque request 106 and the actual regenerative braking torque may be ramped to non-steady state values and may change as a demand for braking and the system constraints change. Furthermore, it should be understood however that the actual regenerative braking 107 could have a value anywhere between zero and the available powertrain regenerative braking torque 102, after the available regenerative braking torque 102 obtains a negative value, based on the need to utilize braking energy or recapture braking energy to charge the battery 20. Accordingly, if the actual regenerative braking torque does have a different value than shown, the friction brake torque request 106 would be adjusted accordingly such that the actual regenerative braking torque and the friction brake toque request 106 would achieve the total brake torque demand 104. Furthermore, it should be understood that the negative value of the lift pedal torque request could be dynamic and different from the value shown in FIG. 5 and the time at which a brake demand starts (t₈) could be adjusted to occur at a different time that shown. If the lift pedal torque request is different or the time of the brake demand is shifted, the friction brake torque request 106 and the actual regenerative braking will be adjusted accordingly based on the description above.

The words used in the specification are words of description rather than limitation, and it is understood that various changes may be made without departing from the spirit and scope of the disclosure. As previously described, the features of various embodiments may be combined to form further embodiments of the invention that may not be explicitly described or illustrated. While various embodiments could have been described as providing advantages or being preferred over other embodiments or prior art implementations with respect to one or more desired characteristics, those of ordinary skill in the art recognize that one or more features or characteristics may be compromised to achieve desired overall system attributes, which depend on the specific application and implementation. As such, embodiments described as less desirable than other embodiments or prior art implementations with respect to one or more characteristics are not outside the scope of the disclosure and may be desirable for particular applications. 

What is claimed is:
 1. A vehicle comprising: a controller programmed to, in response to releasing an accelerator pedal, decrease a constrained wheel torque request prior to, during, and after a period of lash crossing where the constrained wheel torque request changes direction from positive to negative, wherein a rate at which the constrained wheel torque request decreases during the period of lash crossing is less than rates at which the constrained wheel torque request decreases during times periods before and after the period of lash crossing, and in response to the constrained wheel torque request becoming negative during the period of lash crossing and a braking torque request having an absolute value that exceeds an absolute value of the constrained wheel torque request, limit regenerative braking to the constrained wheel torque request during the period of lash crossing such that the regenerative braking does not satisfy the braking torque request, and increase friction braking torque to satisfy a remainder of the braking torque request.
 2. The vehicle of claim 1, wherein the controller is further programmed to, in response to initiation of the braking torque request prior to the decreasing constrained wheel torque request becoming negative, prevent regenerative braking until the constrained wheel torque request becomes negative.
 3. A vehicle system comprising; a controller programmed to, in response to releasing an accelerator pedal, decrease a filtered demanded wheel torque request prior to, during, and after a period of lash crossing where the filtered demanded wheel torque changes direction from positive to negative, and in response to the filtered demanded wheel torque request becoming negative during the period of lash crossing and a braking torque request having an absolute value that exceeds an absolute value of the filtered demanded wheel torque request, limit actual regenerative braking torque to the filtered demanded wheel torque during the period of lash crossing such that the actual regenerative braking does not the braking torque request and increase friction braking torque to satisfy a remainder of the braking torque request.
 4. The system of claim 3, wherein the controller is further programmed to, in response to initiation of the braking torque request prior to the decreasing filtered demanded wheel torque becoming negative, prevent regenerative braking until the filtered demanded wheel torque becomes negative.
 5. The system of claim 4, wherein the filtered demanded wheel torque is reduced according an anti-jerk torque schedule.
 6. The vehicle of claim 1, wherein the controller is programmed to increase regenerative braking to the constrained wheel torque request during the period lash crossing subsequent to the constrained wheel torque request becoming negative.
 7. The vehicle of claim 1, wherein the controller is programmed to, in response to increasing a rate at which the constrained wheel torque request decreases and the braking torque request having an absolute value that that exceeds an absolute value of the constrained wheel torque request during a period subsequent to the period of lash crossing, limit regenerative braking to the constrained wheel torque request during the period subsequent to the period of lash crossing such that the regenerative braking does not satisfy the braking torque request, and increase friction braking torque to satisfy a remainder of the braking torque request.
 8. The vehicle of claim 7, wherein the controller is programmed to increase regenerative braking torque to the constrained wheel torque request during the period subsequent to the period of lash crossing.
 9. The vehicle of claim 7, wherein the controller is programmed to, in response to a completion of the period of the lash crossing, ramp the constrained wheel torque request to an unfiltered demanded wheel torque during the period subsequent to the period of lash crossing.
 10. The vehicle of claim 1, wherein the constrained wheel torque request is reduced according to an anti-jerk torque schedule.
 11. The vehicle of claim 3, wherein the controller is programmed to increase regenerative braking torque to the filtered demanded wheel torque request during the period of lash crossing subsequent to the filtered demanded wheel torque request becoming negative.
 12. The vehicle system of claim 3, wherein the controller is programmed to decrease the filtered demanded wheel torque request at a first rate during a period prior to the period of lash crossing and decrease the filtered demanded wheel torque request at a second rate during the period of lash crossing, wherein the first rate is greater than the second rate.
 13. The vehicle system of claim 12, wherein the controller is programmed to decrease the filtered demanded wheel torque request at a third rate during a period that is subsequent to the period of lash crossing, wherein the third is greater than the second rate.
 14. The vehicle system of claim 13, wherein the controller is programmed to increase regenerative braking torque to the filtered demanded wheel torque request during the period that is subsequent to the period lash crossing.
 15. The vehicle system of claim 13, wherein the controller is further programmed to, in response to a completion of the period of the lash crossing, ramp the filtered demanded wheel torque request at the third rate to an unfiltered demanded wheel torque during the period that is subsequent to the period lash crossing. 